Studies will be designed to elucidate how unsensitized mononuclear leukocytes obtained from the peripheral blood of healthy individuals recognize and kill tumor cells. The majority of the studies will be carried out with large granular lymphocytes (LGL) or a phenotypically defined subset, natural killer (NK) cells. Some experiments will utilize monocytes. So far, the "recognition" component of this phenomenon has been more resistant to analysis than the lysis step. Recognition involves overlapping concepts: Effectors may have to preferentially seek out their targets; next, they must adhere to the targets because cytolysis is known to be dependent on conjugation. Moreover, conjugation alone does not necessarily result in lysis. Thus, three aspects of "recognition" will be addressed. In order to learn how sensitive tumor cells differ from resistant targets, we shall carry out parallel studies on various target cell panels: 1) a panel of melanoma cell lines, which are known to be either NK sensitive (NKs) or NK resistant (NKr), 2) fetal and transformed fibroblasts (NKs) and normal adult fibroblasts (NKr), 3) cell lines which are NK(s) like K562, U937 and HL60 and which can be induced to differentiate when they become NK(r), and 4) cells which are NK(r) but can be rendered NK(s) in vitro, e.g., by treatment with phorbols. Comparable target cells and their isolated membranes will be subjected to biochemical, histochemical, immunochemical, ultrastructural and freeze-fracture analyses to pinpoint the target structure(s) responsible for NK cell recognition. Among candidate molecules to be studied are: fibronectin, laminin, transferrin, and hemopoxin, as well as the receptors for these proteins. Isolated receptors which may constitute the "recognition" units on the target membrane and which may be "shed" into the microenvironment of tumor cells, will also be used in chemotaxis assays. A variety of well defined tumor cell antigens to which monoclonal antibodies are available will be used similarly. In addition, we hope to determine on the ultrastructural level, by means of freeze-fracture and "label-fracture" techniques, where in the membrane these structures are located and whether they are lost, translocated or "covered up" when there is a change from NK(s) to NK(r) or vice versa. An understanding of the mechanism of spontaneous tumor cytolysis is obviously necessary before modulation of effector or target cells aimed at a therpeutic intervention of neoplastic disease in man can be envisioned.